(a) Field
The described technology relates generally to a frame transmitting method and a transmission mode detecting method. More particularly, the described technology relates generally to a frame transmitting method and a transmission mode detecting method in a wireless local area network (WLAN).
(b) Description of the Related Art
A WLAN is being standardized by the IEEE (Institute of Electrical and Electronics Engineers) Part 11 under the name of “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.”
After an original standard was published on 1999, new version standards are continuously published by amendments. The IEEE standard 802.11a (IEEE Std 802.11a-1999) supporting 2.4 GHz band was published on 1999, and the IEEE standard 802.11g (IEEE Std 802.11g-2003) supporting 5 GHz band was published on 2003. These standards are called legacy. Subsequently, the IEEE standard 802.11n (IEEE Std 802.11n-2009) for enhancements for higher throughput (HT) was published on 2009, and the IEEE standard 802.11ac (IEEE 802.11ac-2013) for enhancements for very high throughput (VHT) was published on 2013. Recently, a high efficiency WLAN (HEW) for enhancing the system throughput in high density scenarios is being developed by the IEEE 802.11ax task group.
In such WLAN scenarios, because new version devices and previous version devices may coexist, the WLAN is being developed for supporting backward compatibility with the previous version devices.
Therefore, the WLAN device detects a transmission mode of the received frame and interprets the frame when the transmission mode is a supportable mode. The WLAN provides an auto detection scheme for detecting the transmission mode of the frame.
In the IEEE standard 802.11a or 802.11g, a symbol of a legacy signal field (L-SIG), which is subsequent to a legacy short training field (L-STF) and a legacy long training field (L-LTF), is modulated using binary phase shift keying (BPSK) modulation. A data field subsequent to the L-SIG is modulated using various modulation schemes from the BPSK to a 64-quadrature amplitude modulation (64-QAM). An HT-mixed format having a legacy structure of the L-STF, the L-LTF, and the L-SIG is provided in the IEEE standard 802.11n. In the HT-mixed format, an HT signal field (HT-SIG) follows the L-SIG, and the HT-SIG is modulated using quadrature binary phase shift keying (QBPSK) having the different phase from the BPSK. In this case, two symbols of the HT-SIG are modulated using the QBPSK modulation. In the IEEE standard 802.11ac, the legacy structure is maintained and a VHT signal field (VHT-SIG) follows the L-SIG. The first symbol of the VHT-SIG is modulated using the BPSK modulation and the second symbol of the VHT-SIG is modulated using the QBPSK modulation.
A WLAN device can determine that a transmission mode of a frame is a mode according to the IEEE standard 802.11n, particularly the HT-mixed format, when the first symbol following the L-SIG is modulated with the QBPSK modulation. The WLAN device can determine that the transmission mode of the frame is a mode according to the IEEE standard 802.11ac when the first symbol following the L-SIG is modulated using the BPSK modulation and the second symbol is modulated using the QBPSK modulation.
Therefore, it is required to determine a transmission mode according to a new version WLAN when the new version WLAN is developed.